1. Field of the Invention
The present invention relates to an optical pick-up head apparatus capable of recording, reproducing, or erasing optical information memorized on a magneto-optical recording medium such as an optical disk or an optical card.
2. Description of the Prior Art
Recently, a magneto-optical disk system for recording information on the magneto-optical recording medium has become popular and has been practiced in various ways. Furthermore, many kinds of optical systems have been proposed as an optical pick-up head apparatus which can correctly read and write information from and into the magneto-optical recording medium.
FIG. 7 is a schematic view showing a constitution of one example of a conventional optical pick-up head apparatus. A spherical beam 7 emitted from a semiconductor laser source 1 enters a collimating lens 30 and, then, is converted into a parallel beam through this collimating lens 30. This parallel beam is further entered into a holographic optical element 52 and, then, a plurality of diffraction beams are generated.
On the forward optical path leading to a magneto-optical recording medium 4 from the semiconductor laser source 1, the 0-order diffraction beam 70 passing through the holographic optical element 52 is utilized for reading or writing signals. The 0-order diffraction beam 70 from the holographic optical element 52 is reflected at a polarized beamsplitter 50 and subsequently is focused on the magneto-optical recording medium 4 by an objective lens 31. Then, the polarization direction of the beam 70 is slightly rotated by virtue of Kerr effect of the magneto-optical recording medium 4.
The beam 70 reflected back and diffracted at the magneto-optical recording medium 4 again passes the objective lens 31 and, in turn, is split into two beams 701 and 702 by means of the polarized beamsplitter 50. The beam 701 having passed through the polarized beamsplitter 50 is further split by a Wollaston prism 51 into two beams 701a and 701b on the basis of the polarization direction of the beam 701.
The beams 701a and 701b are respectively converted by a converging lens 32 into converging beams, which are received by a photodetector unit 81 having two photodetectors 811 and 812. Information recorded on the magneto-optical recording medium 4 is obtained through a differential operation which performs mathematical functions on the difference between two output signals generated from photodetectors 811 and 812.
On the other hand, the beam 702 being reflected at the polarized beamsplitter 50 is entered into the holographic optical element 52 and, then, 1-order diffraction beams 702a and 702b are generated. These 1-order diffraction beams 702a and 702b are converted by a lens 30 into converging beams, which are received by a photodetector unit 82.
FIG. 8 shows photodetectors 821.about.824 of the photodetector unit 82 and their receiving images of the 1-order diffraction beams 702a and 702b supplied from the holographic optical element 52. FIG. 8(b) shows a focusing condition wherein the beam 7 emitted from the semiconductor laser source 1 accurately focuses on the magneto-optical recording medium 4. To the contrary, FIGS. 8(a) and 8(c) show defocusing conditions wherein the images are defocussed in opposite directions.
The focusing-error signal can be obtained by carrying out the differential operation based on the summation of output signals from the photodetectors 821 and 824 and the summation of output signals from the photodetectors 822 and 823. The tracking-error signal can be obtained by carrying out the differential operation based on the summation of output signals from the photodetectors 821 and 822 and the summation of output signals from the photodetectors 823 and 824. These methods are well-known as "double-knife edge method" or "push-pull method", respectively, and disclosed in detail, for example, in the U.S. Pat. No. 4,665,310.
FIG. 9 shows one example of intensities of the information signal detected by the photodetector unit 81 and the focusing (or tracking)-error signal detected by the photodetector unit 82 relative to the diffraction efficiency of the 0-order diffraction beam from the holographic optical element 52. The focusing (or tracking)-error signal becomes maximum when the diffraction efficiency of the 0-order diffraction beam from the holographic optical element 52 is 50%. On the other hand, the information signal becomes maximum when the diffraction efficiency of the 0-order diffraction beam from the holographic optical element 52 is 100%.
Namely, it is impossible to realize the signal detection capable of optimizing both the focusing (or tracking)-error signal and the information signal. In other words, it is inevitable that either of these signals is deteriorated.